Java Fern (Microsorum pteropus) in two forms

Java Fern (Microsorum pteropus). This amphibious Polypody  fern is widely distributed throughout tropical southeast Asia, northwards to southern China, and throughout Indonesia to the Philippines. It grows as an epiphyte that's said to thrive out of water, as long as it's in a spray zone and constantly wet. Nevertheless, I find that even in the humid air under a glass top Microsorum's emerse leaf tips blacken. Quite tolerant of moderate light, with stronger lighting Java Fern can also adapt to hard water and even to some salt. It's the Rift Lake cichlid keeper's mainstay. Two main types that are morphologically quite consistent across a range of growing conditions are distinguished: a robust form and a gracile form. 
 
It is "MY-kro-SOR-um," by the way, not spelled or pronounced "MY-kro-SOR-ee-um." Microsorum means "tiny spore," a name that would apply to pretty much any fern, I'd think; pteropus ("terr-OH-pus") refers to the "winged base" of vigorously growing leaves on a well-established rhizome of the robust form. 
 
The robust form of Java Fern is larger in every part, with broad undivided fronds up to 18 inches long, often with a lobed or trilobed base, of a bright mid-green, very puckered and quilted between the side veins, with ruffled edges. This familiar robust form has fronds spaced more widely along the scaly running rhizome. Its brown sporangia are scattered over the undersides.The robust form  is very plastic. Some forms of it seem pretty stable, like the two sports discovered in one of their greenhouses in 1991 and distributed by the Danish aquatic plant firm Tropica, one named "Windeløv" for their founder and the other named "Tropica." The US Dept. of Agriculture protects our Florida aquatic plantfarmers by forbidding imports, but a mutation very like "Tropica" is in the American trade, offered as "Java Lace Fern."
 
Genetic mutations aren't the only causes of changes. At other times, shifting parameters of pH, carbonate hardness, availability of nutrients and light intensity combine to reshape the form of Java Fern. 
 
The other, gracile form is much smaller, with narrower (less than ¾ inch across), lanceolate darker green leaves that aren't so ruffled, which form a denser, more clumped growth. Its spore cases are neatly arranged in rows flanking the midrib. Tropica distributes a form like this one as Microsorum species "narrow." Some plant pros, like Dave Gomberg, suggest that this smaller variant is actually Microsorum blassii, found in New Guinea and nearby islands at the edges of small lowland streams, often growing completely submerse. Christel Kasselmann, in Aquarium Plants (2003), treats the two forms as a single species.
 
Sometimes you may find minute plants growing from the sporecases, especially as an over-mature leaf slowly distintegrates. But Java Fern reproduces itself more vigorously from the adventitious plantlets that develop along the outward edges of mature leaves. In time they can be gently separated from the leaf, rootlets and all. And it also reproduces, less prolifically, in the more conventional way, with offsets from the running rhizome, which is the main green root along which the leaves sprout.
 
When the plantlets sprouting from overmature leaftips have three or four well-developed leaves and some trailing roots, I separate them. I fold a plant weight-strip in half like a bobby pin and thread the plantlets onto one leg. I crimp it lightly and set the group down where it will get plenty of light but I can't see it: a watched pot never boils. Undisturbed and weighted, the young Java Ferns develop in a nice orderly way, fronds all rising from the rhizome in the same direction, hopefully. Months later I'll separate the juveniles and attach them, two and three at a time, to coconut shells or pumice rock.
 
The one trick to success with Java Fern, everyone agrees, is not to bury that rhizome. When I've inadvertently buried a rhizome of the gracile form in fine gravel, the leaves that continued to sprout from it became further dwarfed to a couple of inches long. Not absolutely invariably such a very bad idea: they have worked well like this for several years in a 10-gallon tank. From the rhizome also spring the copious brown thready roots, which act more as holdfasts for the plant than as conduits for nutrients.  Since much of Microsorum's nutritional uptake is through the roots, leaving them untrimmed on the robust form encourages mature, winged leaves up to 18 inches tall. A reason for having tall tanks, in my opinion. If you need less than gigantic leaves, in a 10-gallon tank perhaps, don't hesitate to shorten the roots with scissors to a convenient length when you are dividing and resetting your Java Fern.
 
The rhizome, its thready roots and its robust leaves can just be tucked behind other plant growth. I was taught not long ago to staple the roots (not the green rhizome) to coconut shell: this gives me a portable Java Fern and a portable coconut shell cave: endless possibilities, eh? And either way  I can lift out the whole unit to cut away any tired-looking leaves. Or to clear away a portable landscape to net a particularly wily fish.
 
In very soft water, where pH has a tendency to drop, Java Fern can undergo a kind of meltdown when pH hits 6.0. In this meltdown, the leaves go translucent, starting at their tips; I've seen it characterized as a disease, but I think it's just a reaction to pH stress. There is also a black rot of mature leaves that can cause trouble: is it made worse by nitrate levels that are too high?
 
Plant nibblers avoid Java Fern, and the compilers of Baensch Aquarium Atlas vol. I reported that some Scats that ate it died. This tale has certainly lost no momentum since: I see it retold endlessly. Even if Microsorum isn't acutely toxic, doubtless there are phenols and suchlike in its leaves that make it an undesirable fish salad.
 
Links.  Microsorum pteropus at Wikipedia. Microsorum pteropus at the AquariumWiki. Microsorum pteropus at AquaticPlantCentral's PlantFinder.